The present invention relates to a speed detecting device for an apparatus used, for instance, to stop the rotary shaft of a machine tool such as a punch or lathe at a predetermined position.
FIG. 1 shows the basic construction of a conventional stopping device. In FIG. 1, reference numeral 1 designates a control unit; 2, an electric motor; 3, a motor shaft; 4 and 5, gears; 6, a rotary shaft which is to be stopped at a predetermined rotary position thereof; 7, a magnet mounted on the rotary shaft 6; 8, a position detector implemented with a magnetic detector; 9, a pulse encoder for producing a speed signal 12; 10, an output signal provided by the position detector; and 11, a three-phase cable.
The operation of the conventional device is as follows: The control unit 1 applies a command to the motor 2 to cause it to selectively rotate the motor shaft 3 so that the rotary shaft 6 is rotated or stopped at a predetermined position through the gears 4 and 5. The position of the rotary shaft is detected by the position detector 8 by detecting the position of the magnet 7 on the rotary shaft 6. The output signal 10 of the position detector 10 is fed back to the control unit 1.
FIG. 2 is a block diagram showing a control system for the conventional stopping device. In FIG. 2, reference numeral 13 designates a position control amplifier; 14, a speed control amplifier; 15, an adder; and 16, a speed command.
In a device using an induction motor as the above-described electric motor, speed control is carried out as follows: A speed control device for reducing the speed to zero at the designated point is used in the stopping operation. More specifically, as shown in FIG. 2, the difference between the speed signal 12 outputted by the pulse encoder 9 and the output signal 10 of the position detector 8 as indicated in a graph in FIG. 3 and which is a target speed value v is calculated to obtain a speed command 16. The speed is then zeroed so as to cause the shaft to stop at the aimed point.
In the speed control device, speed control must be performed with high precision, especially when the shaft is near the designated stopping point, and therefore it is essential to obtain an accurate speed signal from the motor shaft. In the conventional device, the speed control system is constructed using the pulse encoder 9 as a detector for obtaining the speed signal. Calculation of the speed with the pulse encoder 9 can be achieved by counting pulses per unit time as illustrated in FIG. 4. When, with a pulse encoder generating N.sub.0 pulses per revolution, n pulses are counted in a period of time T, the speed is represented by (1/N.sub.0).multidot.(n/T). In this calculation, the resolution of speed detection is (1/N.sub.0).multidot.(n/T) per pulse, and as indicated by waveform (A) and (B) in FIG. 4 which show pulse encoder output signals during low speed operation, an error of one pulse is caused by the difference of sampling times. Thus, the conventional device is disadvantageous in that the error is large in the range of speed which is beyond the resolution of speed detection, and the accuracy of speed control is low in the range of low speeds typically involved in stopping operations. In order to eliminate these difficulties, the number N of pulses per revolution produced by the pulse encoder 9 may be increased thereby to increase the resolution of speed detection. However, this approach is disadvantageous in that the device becomes expensive because a precision pulse encoder must be used, and in order to count pulses without an overflow during high speed operation, the number of places in the counter must be increased, with the result that the control circuit is made more intricate and expensive.